1. Technical Field
The inventions generally relate to producing crude oil or natural gas from a well drilled into a subterranean formation. More particularly, the invention is directed to methods of separating coal fines from produced water, which can include water from a drilling fluid, a treatment fluid, or naturally occurring oilfield water.
2. Background Art
Producing Oil & Gas
Petroleum is a complex mixture of naturally-occurring hydrocarbon compounds found in the rock of certain subterranean formations. As used herein, a “hydrocarbon compound” or simply “hydrocarbon” broadly refers to an organic compound comprising hydrogen and carbon. As used herein, a “hydrocarbon” does not necessarily consist entirely of hydrogen and carbon and it is not necessarily a naturally occurring compound but can be synthetic. A hydrocarbon compound can be as simple as the methane (CH4) molecule or can be a highly complex molecule or anything in between. For example, a hydrocarbon molecule can have the shape of a chain, a branched chain, a ring, or a more complex structure. Depending on the chemical formula and structure, a hydrocarbon compound is a gas, liquid, or solid at room temperature and 1 atmosphere pressure.
In general, an “organic compound” is a chemical compound containing a carbon-hydrogen bond (C—H), but a compound containing a carbon-halide bond (C—X) is also considered to be an organic compound (e.g., “organohalides”). Thus, the term “organic compound” or “organic” includes a “hydrocarbon compound” but is broader than “hydrocarbon.”
Depending on the composition of the petroleum, it is a gas, liquid, or solid (when at room temperature and 1 atmosphere pressure), but the term is generally used to refer to liquid crude oil. In the context of producing petroleum from a well, “oil” and “gas” refers to crude oil and natural gas. Impurities such as sulfur, oxygen, and nitrogen are common in petroleum. There is considerable variation in color, specific gravity, odor, sulfur content, and fluid viscosity in petroleum from different areas.
A subterranean formation containing oil, gas, or both is sometimes referred to as a reservoir. A reservoir is in a shape that will trap hydrocarbons and that is covered by an impermeable or sealing rock. A reservoir may be located under land or under the seabed off shore. Oil and gas reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs) below the surface of the land or seabed.
As used herein, “subterranean formation” refers to the fundamental unit of lithostratigraphy. A subterranean formation is a body of rock that is sufficiently distinctive and continuous that it can be mapped. In the context of formation evaluation, the term refers to the volume of rock seen by a measurement made through the wellbore, as in a log or a well test. These measurements indicate the physical properties of this volume, such as the property of permeability. A “zone” refers to an interval or unit of rock along a wellbore that is differentiated from surrounding rocks based on petroleum content or other features, such as faults or fractures. A “zone” can be, for example, a portion of a subterranean formation along a wellbore that has been fractured, acidized, or otherwise treated.
To produce oil or gas from a reservoir, a wellbore is drilled into a subterranean formation, which may be a reservoir or adjacent to a reservoir. The “wellbore” refers to the drilled hole, including the openhole or uncased portion of the well. As used herein, the “borehole” refers to the inside diameter of the wellbore wall, that is, the rock face or wall that bounds the drilled hole. A wellbore can have vertical and horizontal portions, and it can be straight, curved, or branched. The wellhead is the surface termination of a wellbore. As used herein, “uphole” and “downhole” are relative to the wellhead, regardless of whether a wellbore portion is vertical or horizontal.
As used herein, a “well” includes at least one wellbore. A “well” can include a near-wellbore region of a subterranean formation surrounding a portion of a wellbore that is in fluid communication with the wellbore. As used herein, “into a well” means at least through the wellhead. It can include into any downhole portion of the wellbore or through the wellbore and into a near-wellbore region.
Oil wells usually produce some gas and usually produce some water, too. Most oil wells eventually produce mostly gas or water.
Well Services and Well Fluids
Generally, well services include a wide variety of subterranean operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, completion, and workover. “Drilling” refers to the events and equipment necessary for drilling a wellbore. “Completion” refers to the events and equipment necessary to bring a wellbore into production once drilling operations have been concluded, including but not limited to the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. “Workover” refers to the performance of major maintenance or remedial treatments on an oil or gas well.
Completion and workover operations may include, but are not limited to, cementing, gravel packing, stimulation, and conformance operations. Many of these well services are designed to facilitate or enhance the production of desirable fluids from or through a subterranean formation.
As used herein, the word “treatment” refers to a treatment for a well or subterranean formation penetrated by a wellbore that is adapted to achieve a specific purpose in completion or workover, such as stimulation, isolation, or conformance control; however, the word “treatment” does not necessarily imply any particular purpose.
Drilling requires the use of a drilling fluid. As used herein, a “drilling fluid” is any of a number of fluids, including fluid mixtures of a liquid with particulate solids or gas (such as suspensions, emulsions, foams) used in operations to drill boreholes into the earth. The term is synonymous with “drilling mud” in general usage, although sometimes the term is used to refer to more sophisticated and well-defined “muds.” One classification scheme for drilling fluids is based on singling out the component that clearly defines the function and performance of the fluid: (1) water-based, (2) non-water-based, and (3) gaseous (pneumatic). Each category has a variety of subcategories that overlap each other considerably.
A treatment typically involves introducing a treatment fluid into a well. As used herein, a “treatment fluid” is a fluid used to resolve a specific condition of a wellbore or subterranean formation. As used herein, a “treatment fluid” also means the specific composition of a fluid at the time the fluid is being introduced into a wellbore. A treatment fluid is typically adapted to be used to achieve a specific purpose, such as stimulation, isolation, or control of reservoir gas or water. The word “treatment” in the term “treatment fluid” does not necessarily imply any particular action by the fluid.
A “spacer fluid” is a fluid used to physically separate one special-purpose fluid from another. A special-purpose fluid can be a drilling fluid, a cementing fluid, or a treatment fluid. Special-purpose fluids are typically prone to contamination, so a spacer fluid compatible with each is used between the two. For example, a spacer fluid is used when changing fluid types used in a well. For example, a spacer fluid is used to change from a drilling fluid during drilling a well to a cement slurry during cementing operations in the well. In case of an oil-based drilling fluid, it should be kept separate from a water-based cementing fluid. In changing to the latter operation, a chemically treated water-based spacer fluid is usually used to separate the drilling fluid from the cement slurry. By way of another example, a spacer fluid can be used to separate two different types of treatment fluids.
As used herein, a “well fluid” refers to any fluid adapted to be used in a well for a particular purpose, without necessarily implying any particular purpose. A “well fluid” can be, for example, a drilling fluid, a cementing fluid, a treatment fluid, or a spacer fluid. As used herein, a “well fluid” means the specific composition of a fluid at the time the fluid is being introduced into a wellbore.
Produced Water
As used herein, “produced water” is water that is produced from or through a wellbore of a well through the wellhead to the surface. Produced water has a continuous phase of water or an aqueous solution. Produced water can include water from a previously introduced well fluid, such as a drilling fluid or a treatment fluid, or water naturally occurring in a subterranean formation. The term often implies an inexactly known or unknown composition. Produced water can include, for example, coal fines, other suspended solids, dissolved inorganic ions, crude oil, or a chemical of a previously introduced well fluid.
Produced water can be characterized is many different ways. Some of the more basic characterizations include, for example, pH, concentration of total suspended solids (“TSS”), concentration of particular types of solids such as coal fines, concentration of total dissolved solids (“TDS”), concentration particular dissolved ions, electrical conductivity, turbidity, and extractable organic compounds such as from crude oil or from a previously introduced well fluid.
The characteristics of produced water can vary widely from one oilfield to another, from one well to another within an oilfield, and over the time of production,. Produced water may have, for example, pH in the range of about 5 to about 9, very high TSS, very high TDS, very high electrical conductivity, very high turbidity, and some extractable organic liquids, usually petroleum.
Problem of Coal Fines in Produced Water
Coal fines are sometimes found in produced water. The coal fines can be naturally occurring in a subterranean formation or they can be created during a drilling operation that penetrates into a coal-bearing formation. The coal fines can be produced with the produced water by the flow of fluid from a subterranean formation or with a returned drilling fluid.
The coal fines prevent the reuse of the produced water because it is undesirable to re-introduce coal fines into the well. Removal of the coal fines would allow the reuse of produced water without the re-introduction of the fines into the wellbore where they can plug pore throats, such as the cleat structure of a coal formation, which would adversely affect production. Removal of the coal fines would also aid in the reduction of wear on surface pumps used in a re-use of the produced water as there would be reduced friction with the absence of solids fed through the pump when fluid is fed down hole.
In addition, a coal flocculation process to remove coal fines preferably should not use a flocculation or coagulation polymer because residual polymer in the treated produced water should not be introduced into a subterranean formation. The use of prior coal fines flocculation technology would inevitably result in the potential for overtreatment with either flocculant or coagulant and recirculation of the overtreatment back into the reservoir. Further, prior flocculation technology requires large proportions of treatment chemicals.